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HAProxy - Load balancer
205f1cbf4c
The fact that the watchdog timer measures the execution time from the last return from the poller tends to amplify the impact of multiple bad tasks, and may explain some of the panics reported by Felipe and Ricardo in GH issues #3084, #3092 and #3101. The problem is that we check the time if we see that the scheduler appears not to be moving anymore, but one situation may still arise and catch a bad task: - one slow task takes so long a time that it triggers the watchdog twice, emitting a warning the second time (~200ms). The scheduler is rightfully marked as stuck. - then it completes and the scheduler is no longer stuck. Many other tasks run in turn, they all take quite some time but not enough to trigger a warning. But collectively their cost adds up. - then a task takes more than the warning time (100ms), and causes the total execution time to cross the second. The watchdog is called, sees that we've spend more than 1 second since we left the poller, and marks the thread as stuck. - the task is not finished, the watchdog is called again, sees more than one second with a stuck thread and panics 100ms later. The total time away from the poller is indeed more than one second, which is very bad, but no single task caused this individually, and while the warnings are OK, the watchdog should not panic in this case. This patch revisits the approach to store the moment the scheduler was marked as stuck in the wdt context. The idea is that this date will be used to detect warnings and panics. And by doing so and exploiting the new is_sched_alive(thr), we can greatly simplify the mechanism so that the signal handling thread does the strict minimum (mark the scheduler as possibly stuck and update the stuck_start date), and only bounces to the reporting thread if the scheduler made no progress since last call. This means that without even doing computations in the handing thread, we can continue to avoid all bounces unless a warning is required. Then when the reporting thread is signaled, it will check the dates from the last moment the scheduler was marked, and will decide to warn or panic. The panic decision continues to pass via a TH_FL_STUCK flag to probe the code so that exceptionally slow code (e.g. live cert generation etc) can still find a way to avoid the panic if absolutely certain that things are still moving. This means that now we have the guarantee that panics will only happen if a given task spends more than one full second not moving, and that warnings will be issued for other calls crossing the warn delay boundary. This was tested using artificially slow operations, and all combinations which individually took less than a second only resulted in floods of warnings even if the total reported time in the warning was much higher, while those above one second provoked the panic. One improvement could consist in reporting the time since last stuck in the thread dumps to differentiate the individual task from the whole set. This needs to be backported to 3.2 along with the two previous patches: MINOR: sched: let's permit to share the local ctx between threads MINOR: sched: pass the thread number to is_sched_alive() |
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HAProxy
HAProxy is a free, very fast and reliable reverse-proxy offering high availability, load balancing, and proxying for TCP and HTTP-based applications.
Installation
The INSTALL file describes how to build HAProxy. A list of packages is also available on the wiki.
Getting help
The discourse and the mailing-list are available for questions or configuration assistance. You can also use the slack or IRC channel. Please don't use the issue tracker for these.
The issue tracker is only for bug reports or feature requests.
Documentation
The HAProxy documentation has been split into a number of different files for ease of use. It is available in text format as well as HTML. The wiki is also meant to replace the old architecture guide.
Please refer to the following files depending on what you're looking for:
- INSTALL for instructions on how to build and install HAProxy
- BRANCHES to understand the project's life cycle and what version to use
- LICENSE for the project's license
- CONTRIBUTING for the process to follow to submit contributions
The more detailed documentation is located into the doc/ directory:
- doc/intro.txt for a quick introduction on HAProxy
- doc/configuration.txt for the configuration's reference manual
- doc/lua.txt for the Lua's reference manual
- doc/SPOE.txt for how to use the SPOE engine
- doc/network-namespaces.txt for how to use network namespaces under Linux
- doc/management.txt for the management guide
- doc/regression-testing.txt for how to use the regression testing suite
- doc/peers.txt for the peers protocol reference
- doc/coding-style.txt for how to adopt HAProxy's coding style
- doc/internals for developer-specific documentation (not all up to date)
License
HAProxy is licensed under GPL 2 or any later version, the headers under LGPL 2.1. See the LICENSE file for a more detailed explanation.